Loops and Control Statements (continue, break and pass) in Python

Python programming language provides the following types of loops to handle looping requirements.

Python While Loop

Until a specified criterion is true, a block of statements will be continuously executed in a Python while loop. And the line in the program that follows the loop is run when the condition changes to false.

Syntax of Python While

while expression:
    statement(s)

In Python, all the statements indented by the same number of character spaces after a programming construct are considered to be part of a single block of code. Python uses indentation as its method of grouping statements. 

Python3

# prints Hello Geek 3 Times
count = 0
while (count < 3):
    count = count+1
    print("Hello Geek")

Output:

Hello Geek
Hello Geek
Hello Geek

See this for an example where a while loop is used for iterators. As mentioned in the article, it is not recommended to use a while loop for iterators in python.  

Python for Loop

In Python, there is no C style for loop, i.e., for (i=0; i<n; i++). There is a “for in” loop which is similar to for each loop in other languages. 

Syntax of Python for Loop

for iterator_var in sequence:
    statements(s)

It can be used to iterate over iterators and a range. 

Python3

# Iterating over a list
print("List Iteration")
l = ["geeks", "for", "geeks"]
for i in l:
    print(i)

# Iterating over a tuple (immutable)
print("\nTuple Iteration")
t = ("geeks", "for", "geeks")
for i in t:
    print(i)

# Iterating over a String
print("\nString Iteration")
s = "Geeks"
for i in s :
    print(i)

# Iterating over dictionary
print("\nDictionary Iteration")
d = dict()
d['xyz'] = 123
d['abc'] = 345
for i in d :
    print("%s %d" %(i, d[i]))

Output:

List Iteration
geeks
for
geeks

Tuple Iteration
geeks
for
geeks

String Iteration
G
e
e
k
s

Dictionary Iteration
xyz  123
abc  345

Time complexity: O(n), where n is the number of elements in the iterable (list, tuple, string, or dictionary).
Auxiliary space: O(1), as the space used by the program does not depend on the size of the iterable.

We can use a for-in loop for user-defined iterators. See this for example.  

Python Nested Loops

Python programming language allows using one loop inside another loop. The following section shows a few examples to illustrate the concept. 

Syntax of Python Nested for Loop

The syntax for a nested for loop statement in Python programming language is as follows:

for iterator_var in sequence:
    for iterator_var in sequence:
        statements(s)
        statements(s)

Syntax of Python Nested while Loop

The syntax for a nested while loop statement in Python programming language is as follows:

while expression:
    while expression: 
        statement(s)
        statement(s)

A final note on loop nesting is that we can put any type of loop inside of any other type of loop. For example, a for loop can be inside a while loop or vice versa. 

Python3

from __future__ import print_function
for i in range(1, 5):
    for j in range(i):
        print(i, end=' ')
    print()

Output:

1
2 2
3 3 3
4 4 4 4

Python Loop Control Statements

Loop control statements change execution from their normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed. Python supports the following control statements. 

Python Continue 

It returns the control to the beginning of the loop. 

Python3

# Prints all letters except 'e' and 's'
for letter in 'geeksforgeeks':
    if letter == 'e' or letter == 's':
        continue
    print('Current Letter :', letter)

Output:

Current Letter : g
Current Letter : k
Current Letter : f
Current Letter : o
Current Letter : r
Current Letter : g
Current Letter : k

Python Break 

It brings control out of the loop.

Python3

for letter in 'geeksforgeeks':

    # break the loop as soon it sees 'e'
    # or 's'
    if letter == 'e' or letter == 's':
        break
print('Current Letter :', letter)

Output:

Current Letter : e

Python Pass 

We use pass statements to write empty loops. Pass is also used for empty control statements, functions, and classes. 

Python3

# An empty loop
for letter in 'geeksforgeeks':
    pass
print('Last Letter :', letter)

Output:

Last Letter : s

Exercise: How to print a list in reverse order (from last to the first item) using while and for-in loops.